Our long-term goal is the development of a better understanding of growth and nutrition of fullterm and premature infants. Quantitative determinations of energy and nutrients intake of each infant for each day of study are combined with serial measurements of anthropometric parameters in studying the effect of alterations in diet composition on food consumption and growth. In the area of studies of factors influencing food consumption, we propose to study energy concentration, protein concentration and sweetness of the diet. By serial determinations of 24-hour energy expenditures (by relating heart beats to results of indirects calorimetry and then accumulating heart beats for 24 hours) and serial measurements of body composition by subject-created perturbation of an electromagnetic field (a rapid, non-invasive method) we shall explore the relation of energy intake and energy expenditure to growth. Direct calorimetry will be used in an attempt to validate indirect calorimetry in the growing infant. The difference in rate of loss of administered stable isotopes (deuterium and 18-0) will be studied to determine the usefulness of this method in determining energy expenditures. The protein requirement of infants will be re-evaluated, particularly in relation to protein intake and protein nutritional status of breast-fed infants. The bioavailabililty of urea, which accounts for about 12% of total nitrogen of human milk, will be determined with the use of [15N] urea. Metabolic balance studies will be employed in study of nutrient interactions of interest in clinical management of small premature infants.